As a conventional technique of this kind, the invention disclosed in JP-A-53110102 is known, for example. According to this invention, there are arranged a plurality of variable displacement hydraulic pumps drivable by a single engine, pressure sensors for detecting delivery pressures from the individual hydraulic pumps, pump displacement controllers for controlling displacements of the individual hydraulic pumps, and a computing circuit for being inputted with signals from the individual pressure sensors, performing a predetermined computation and then outputting signals, which correspond to the results of the computation, to the pump displacement controllers. The computing circuit is designed such that the signals from the individual pressure sensors are added, a voltage value equivalent to the sum of outputs predetermined for the individual hydraulic pumps is divided by the added value, and the results of the division is outputted to the pump displacement controller via a limiter circuit.
According to the conventional technique constructed as described above, the output signal to the pump displacement controller is controlled based on signals from the respective pressure sensors such that the total of input torques to the individual hydraulic pumps does not exceed output force power which the engine can output. According to this conventional technique, the sum of input torques to the hydraulic pumps is limited so that, even when any one or more of the plural hydraulic pumps becomes or become higher in delivery pressure, the sum of the input torques to the hydraulic pumps does not exceed the output force power which the engine can output. This conventional technique, therefore, makes it possible to avoid an engine stall and also to use power of the engine rather effectively.
As another conventional technique, the invention disclosed in JP-A-05126104 is also known. This publication discloses a hydraulic circuit for a construction machine, which is equipped with two variable displacement hydraulic pumps and one fixed displacement hydraulic pump and feeds pressure oil from the fixed displacement hydraulic pump to a revolving hydraulic motor. A delivery pressure from the fixed displacement hydraulic pump is guided to regulators for the two variable displacement hydraulic pumps through a restrictor.
The hydraulic circuit disclosed as another conventional technique as mentioned above is designed such that, when the delivery pressure from the fixed displacement hydraulic pump increases, the regulators for the two variable displacement hydraulic pumps operate to reduce the delivery rates from the two variable displacement hydraulic pumps. Owing to this design, the sum of input torques to the individual hydraulic pumps does not exceed force power which an engine can output, so that the engine is protected from an overload.
In the above-described conventional art disclosed in JP-A-53110102, the delivery rates of the plural hydraulic pumps are all controlled evenly so that pressure oil cannot be fed preferentially to any particular actuator even when its flow rate is desired to remain unchanged. In a hydraulic excavator as an illustrative construction machine, a revolving load pressure during revolving drive becomes much higher than load pressures to hydraulic cylinders which drive front members such as a boom, an arm and a bucket. Upon combined operation of one or more members and a revolving hydraulic actuator, it is desired to feed pressure oil preferentially to the revolving hydraulic motor rather than the hydraulic cylinders for the front members. This is particularly so during initial operation of the revolving drive. According to this conventional technique, however, all the hydraulic pumps are designed to be controlled evenly, so that during such combined operation, the feed of pressure oil to the revolving hydraulic motor becomes insufficient and the revolving speed becomes slower. When the load pressure on the hydraulic cylinder for driving one of the front members changes during combined operation of the front members and the revolving hydraulic motor, the flow rate of pressure oil to be fed to the revolving hydraulic motor varies so that the revolving speed changes. During operation of a hydraulic excavator, variations especially in revolving speed make its operator feel extreme unpleasant. As appreciated from the foregoing, no consideration is made to any particular actuator in this conventional technique, and therefore, a problem exists especially in operability.
In the other conventional technique disclosed in JP-A-05126104, on the other hand, the fixed displacement hydraulic pump is used as a source of pressure oil to the revolving motor. During combined operation of the revolving hydraulic motor and another actuator, variations in the load on the actuator hence does not affect the revolving speed. To prevent the sum of input torques to the individual hydraulic pumps from exceeding the output force power which the engine can output, however, the conventional technique is designed to decrease the input torques to the remaining, two variable displacement hydraulic pumps. When the revolving load becomes greater during revolving drive of a hydraulic excavator, the delivery pressure from the fixed displacement hydraulic pump becomes extremely high, and the delivery rates from the remaining, two variable displacement hydraulic pumps are substantially decreased. When revolving drive is performed during operation of a boom, for example, the flow rate of pressure oil to be fed to the hydraulic cylinder for the boom extremely decreases, leading to a sudden slowdown in the operation speed of the boom. As appreciated from the foregoing, the other conventional technique also involves an unsolved problem especially in operability.
The present invention has been completed in view of the above-described problems of the respective conventional techniques. The present invention, therefore, has as a first object the provision of a hydraulic circuit for a construction machine, which uses three variable displacement hydraulic pumps and makes it possible for one of these hydraulic pumps to feed pressure oil at a stable flow rate to a particular actuator without being affected by torques consumed by the remaining two hydraulic pumps and hence, to smoothly perform driving of the particular actuator.
Further, the present invention has as a second object the provision of a hydraulic circuit for a construction machine, which, even when a load on a particular actuator fed with pressure oil from a third hydraulic pump increases, delivery rates of a first and second hydraulic pumps are not extremely decreased to prevent actuators other than the particular actuator from undergoing an excessive drop in speed and hence, to assure good operability.